Conventional processes for the recovery of high purity aromatic hydrocarbons such as benzene, toulene and xylenes (BTX) from various hydrocarbon feedstocks including catalytic reformate, hydrogenated pyrolysis gasoline, etc., utilize an aromatic selective solvent. Typically, in the practice of such processes, a hydrocarbon feed mixture is contacted in an extraction zone with an aqueous solvent composition which selectively dissolves the aromatic components from the hydrocarbon feedstock, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons, and an extract phase comprising solvent having aromatic components dissolved therein.
A common problem in many aromatics extraction processes is that the extract phase contains, in addition to aromatic hydrocarbons, solvent and contaminating amounts of non-aromatic hydrocarbons from which the aromatics must be recovered. Accordingly, a variety of methods have been proposed to recover the extracted aromatic hydrocarbons while avoiding solvent losses in the aromatic extract product. These methods typically utilize extractive distillation with steam stripping in order to remove the contaminating non-aromatic hydrocarbons from the extract phase followed by another step, e.g., distillation, rectification, washing, in order to remove the solvent from the aromatic hydrocarbons. Several types of recovery methods are hereinafter described.
U.S. Pat. No. 3,590,092 discloses a method for aromatic hydrocarbon recovery that utilizes a single column wherein a side-cut vapor fraction comprising aromatic hydrocarbons and a minor quantity of solvent is withdrawn and introduced into a separate rectifying zone maintained under rectifying conditions, to provide a relatively solvent free aromatic extract product. The side-cut aspect of the method is set forth at col. 6, lines 10 to 43, wherein it is stated that:
"A side-cut vapor fraction is withdrawn via line 25 and passed into rectifying column 32 which is maintained under rectification conditions. Typically, the conditions maintained in rectifying column 32 include a temperature from 200.degree. F. to 250.degree. F. and a pressure from 1 to 10 psig. An overhead stream comprising the desired aromatic hydrocarbons, benzene and toluene, is withdrawn via line 26, condensed in condensor-separator 30, and passed out of the system via line 28. Any water condensed and removed in separator 30 is also removed from the system via line 27. A bottoms fraction comprising primarily sulfolane solvent and heavy aromatic hydrocarbons of the C.sub.10, C.sub.11 and C.sub.12 type (if any) is withdrawn via line 33 and in a preferred embodiment of this invention passed into the lower portion of stripper column 17. Alternatively, a portion of the material in line 33 or all of the material in line 33, if desired, may be passed via line 34 into line 12 as lean solvent suitable for reuse in the extraction zone. Therefore, it can be seen that the lean solvent which in the preferred embodiment of this invention is returned to extractor column 11 via line 12 comprises solvent obtained from the bottom of stripper column 17 and from the bottom of rectifying column 32. A portion of the high purity aromatic hydrocarbon stream in line 28 is diverted via line 31 and passed into the upper section of rectifying column 32 as reflux thereon. PA0 Returning now to stripper column 17, a bottoms fraction comprising lean sulfolane solvent suitable for reuse in the extraction zone is withdrawn via line 12 and in a preferred embodiment of this invention is returned to the upper section of extraction zone 11 as lean solvent in the manner previously discussed. Make-up solvent as needed may be introduced into the system via line 13." PA0 "The second vapor fraction comprising aromatic hydrocarbons, water and solvent which was withdrawn from an intermediate section of the stripping zone is then passed to a lower section of a rectification zone to separate therein the aromatic hydrocarbons from the sulfolane solvent. This separation is accomplished by maintaining the rectification zone under conditions including a temperature of about 100.degree. F. to about 400.degree. F. and a pressure of about 50 mm. Hg to about 25 psig, preferably 5 psig to about 20 psig, and withdrawing from an upper section of the rectification zone a vapor fraction relatively free of solvent comprising aromatic hydrocarbons and water (steam). This vapor fraction is condensed and the aromatics recovered are relatively free of non-aromatics and sulfolane solvent. At least a portion of the liquid water formed when the vapor fraction was condensed is passed to the upper section of the rectification zone to help effectively remove the sulfolane solvent from the aromatics. Withdrawn from a lower section of the rectification zone is a liquid stream comprising solvent and water. This stream is then passed, as reflux, to an intermediate section of the stripping zone to recover therein the sulfolane solvent for use in the extraction zone."
The above-described method utilizes a single distillation column, plus a separate rectifying zone, to recover the aromatic hydrocarbons and avoid solvent losses. However, because the bottoms fraction from the rectifying zone is returned to the lower portion of the stripper column, or alternately bypassed entirely, and then passed with bottoms from the stripper column to the top (raffinate end) of the extraction column, aromatic hydrocarbons present in the bottoms from the rectifying zone can be lost in the extractor raffinate, thereby reducing product recovery.
U.S. Pat. No. 3,702,295 discloses a method for aromatic hydrocarbon recovery that also utilizes the single column, vapor side-cut approach. However, this method differs from that disclosed in U.S. Pat. No. 3,590,092 in that the rectification zone is refluxed with the aqueous phase from the overhead condensate, instead of the hydrocarbon phase. Also, the bottoms fraction from the rectification zone is introduced to an intermediate section in the stripped column instead of the lower section. The side-cut aspect of this method is set forth at col. 6, lines 45 to 68 wherein it is stated that:
The above-described method utilizes a single distillation column, plus a rectification zone which can be incorporated into the single column, to recover the aromatic hydrocarbons and avoid solvent losses. However, because the bottoms fraction from the rectification zone is introduced to an intermediate section of the stripper column, no stripping benefit is obtained from it in the lower section of the stripping zone. Additional steam is supplied to the lower section of the stripper column for this purpose.
Processes other than the vapor side-cut type have also been proposed. For example, U.S. Pat. No. 3,714,003 discloses a process wherein the side-cut vapors are condensed and water-washed to remove solvent from the aromatic hydrocarbons. This process avoids the use of a rectification zone but requires the use of appropriate water-washing equipment such as extractors and mixer-settlers. This type of process can provide a particularly effective means of recovering aromatic hydrocarbons when certain aromatic-selective solvents, e.g., polyalkylene glycols, are used. The use of other solvents having a higher affinity for aromatic hydrocarbons, e.g., certain glycol ethers, may require further processing to avoid solvent losses.
U.S. Pat. No. 4,419,226 discloses a process that utilizes two distillation columns without a side-cut stream for the recovery of aromatic hydrocarbons and a non-aromatic raffinate stream from a hydrocarbon charge stock. The hydrocarbon charge stock is treated with an aromatics-selective solvent to provide an aromatics-rich solvent stream and a non-aromatic raffinate stream. The aromatics- rich solvent stream is treated in a stripper column at conditions to separate substantially all of the non-aromatic hydrocarbons therefrom. The rich solvent stream is subjected to steam stripping to provide a high purity aromatics stream and an aqueous stream comprising the steam condensate and solvent.
The above-described processes set forth various methods for recovering aromatic hydrocarbons from the extract phase from solvent extraction processes. The two distillation column approach offers sufficient design flexbility to accommodate most solvent-aromatic hydrocarbon combinations. Nonetheless, improved methods are sought for the recovery of extracted aromatic hydrocarbons that utilize a single column, side-cut approach and effectively avoid solvent losses.